Add pandoc as an alternative renderer

[DannyBen]

Due to the fact that all semi-native Ruby libraries for markdown generation have issues and long-standing bugs, this PR adds pandoc as an alternative rendering engine. The default is still going to be Redcarpet for the time being, since pandoc requires an external installation, which is less convenient.

To use pandoc as the renderer, add this to .madness.yml:

# ./madness.yml
renderer: pandoc

Notable advantages of using pandoc:

1. Headers that contains diacritics have their ids (for Table of Contents links) generated correctly
   • HTML ids are generated without diacritics and links do not work #158
   • Letters with diacritics (accents) are completely removed in TOC IDs vmg/redcarpet#739
2. Nested lists (3 or more levels deep) are generated correctly
   • ToC levels for not autogenerated ToC are not honored #178
   • Missing List Indent for nested list item vmg/redcarpet#736
   • Bug with multiple nested lists vmg/redcarpet#711

Notable disadvantages of using pandoc:

1. Requires the command pandoc to be installed (usually it is as simple as installing it with your package manager)
2. Does not handle some less common text decorations (like ==highlight==).